Electronic packages are devices that provide connection and communication between "chips" located inside the package and devices located outside of the package. Often, these packages are hermetically sealed to protect the chips from contaminants in the surrounding environment. The outer walls of the hermetic package are typically fabricated from a metallic material, such as KOVAR.TM., (Westinghouse Electric Corp., E. Pittsburgh, Pa. 15235), an iron-nickel-cobalt alloy having a thermal expansion coefficient close to that of hard glass.
Hermetic metal packages must be capable of accommodating electrical communication through the walls of the package with little or no interference. This creates a design problem because the electrical communication lines must be sufficiently insulated from the metal walls of the package to avoid any type of interference. At the same time, the hermeticity of the package must be maintained to protect the internal components.
Traditionally, glass seals have been utilized to insulate conductors from the conductive walls of hermetic packages. Matched glass seals are widely used and much effort has been directed to improving their reliability.
The electronics industry is now able to put more devices on a chip to form integrated circuits, and able to put more chips, integrated circuits, and passive components into a single hermetic package to form a "hybrid" circuit. These hybrid circuits require large packages with high lead counts, i.e. a large number of input and output leads. These leads must be well insulated from the package walls, and also must be well insulated from each other. The leads often have a very fine pitch, that is, they are spaced closely adjacent to each other, which compounds the problem.
Glass seals are often an ineffective way to insulate conductor paths in these devices. Glass seals, particularly glass bead seals, are susceptible to cracks that can ruin the hermeticity of the package. If the probability of a crack is about 1 in 2000 seals, the impact on acceptable yields for a three lead device having three glass seals is relatively minor. But if the package requires 100 or more leads, as is often the case on currently produced hybrid circuits, then this same crack probability will significantly lower the yield of acceptable packages.
One solution to this problem is the use of a multilayer, co-fired ceramic feedthrough. This type of feedthrough has a much higher survivability rate through the normal assembly, testing, handling, and product life. Further, polycrystalline ceramic is much stronger than glass. This construction can also utilize many refined manufacturing techniques for multilayer ceramics.
A hermetic package designed to receive a ceramic feedthrough is typically constructed with a thermally conductive floor, e.g., copper-tungsten or molybdenum, and a metal wall with slots into which the feedthrough is brazed or soldered to provide a hermetic seal. Alternatively, the floor and walls can be fabricated from a single piece that is obtainable using powder metallurgy sintering techniques, in which case the slot for the feedthrough is molded into the case. In all instances, the feedthrough requires a band of metallization completely wrapped around the periphery of the feedthrough to form a continuous surface suitable for a hermetic braze/solder seal.
The production of feedthroughs for hermetic packages thus requires the ability to form "wraparound" metallization. The wraparound metallization provides a continuous braze-wettable surface around the feedthrough. The feedthrough can then be inserted into the package so that the wraparound metallization is surrounded by the slot in the wall of the package, enabling the feedthrough to be brazed into place, forming a substantially hermetic seal.
U.S. Pat. No. 4,416,156 by Demark et al., issued Nov. 22, 1983, discloses an electrical feedthrough for a high pressure housing that includes a ceramic circuit assembly having a metallized bonding area on the surface of the ceramic that enables the ceramic assembly to be brazed to an outer support housing. The bonding area is in the form of a collar for bonding the ceramic feedthrough to the pressure housing to provide a fluid tight seal therebetween. There is no disclosure as to how the bonding area collar is formed around the ceramic.
U.S. Pat. No. 4,487,999 by Baird et al., issued on Dec. 11, 1984, discloses an all-metal microwave chip carrier with ceramic feedthroughs, wherein the feedthroughs are configured to function as coaxial cables having predetermined impedances. The feedthroughs are formed by providing ceramic tubing metallized inside and out, in which the ends are cut away to provide half cylindrical bonding pads.
The upper and lower planar surfaces of a feedthrough, or any similar article, can be metallized by conventional screen-printing techniques while the substrate material is in the green (unfired) state. However, the vertical surfaces cannot be metallized until they are exposed by cutting, breaking or punching the ceramic card or sheet from which they are formed. The side metallization thus requires the additional manufacturing step such as screen printing the vertical sides of the part in either the fired or unfired stage.
Often, the side metallization must be applied manually, which is a labor-intensive, high-cost processing step. Since a single card can contain hundreds of small feedthroughs, each of which must be metallized on two sides, this additional step presents a major handling problem. The advantages of array processing (i.e. forming large numbers of small parts in one array) are therefore lost. Further, when the side metallization is applied by the above-identified techniques, the feedthrough edges cannot be subsequently ground or machined to reach a final dimension, since this would tend to remove the thin metallization layer.
It would be advantageous if the additional manufacturing step of applying side metallization could be eliminated. It would be particularly advantageous if the side metallization could be formed during conventional process steps and permit the use of array processing. It would also be advantageous if the part having the side metallization could be machined after applying the side metallization without completely removing the side metallization.